Water-absorbing resin (super absorbent polymer or SAP) is a water-swellable, water-insoluble polymer gelling agent. Water-absorbing resin is widely used especially for disposable items, including absorbent articles such as disposable diapers and sanitary napkins, agricultural and horticultural water retaining agents, industrial waterproofing agents and the like. Of such water-absorbing resins, particularly a polyacrylic acid (salt)-based water-absorbing resin, in which acrylic acid and/or a salt thereof are/is used as a monomer(s), is most widely used for industrial purposes from the viewpoint of its high water absorption performance.
Disposable diapers, which are one of the main applications of water-absorbing resin, have undergone advances in terms of performance. These advances created demands for many functions (physical properties) of water-absorbing resin. Specific examples of the physical properties of a water-absorbing resin encompass centrifuge retention capacity, saline flow conductivity, absorption ability under load, and a water-soluble component amount. Therefore, a large number of proposals have been made for surface-crosslinking techniques and for changes of, for example, additives and production processes.
For example, Patent Literature 1 discloses a water-absorbing resin which contains, as a repeating unit of a main chain, a water-soluble unsaturated monomer containing an acrylic acid and/or a salt thereof in an amount of not less than 90 mol % and which has a crosslinked structure therein. The water-absorbing resin has an intrinsic viscosity (IV) which is not more than 7.3 (dL/g) with respect to a weight average molecular weight Log (Mw) of 6.10 after a certain treatment.
It is further demanded that absorption performance not deteriorate for a long period of time during actual use of a disposable diaper. Under the circumstances, techniques for preventing deterioration of a water absorbing agent (i.e. making the water absorbing agent more urine resistant) have been proposed. Examples of such techniques disclosed encompass: a technique in which a metal chelate is added while a method for producing a water-absorbing resin is carried out (Patent Literatures 2 through 5); a technique in which a water-absorbing resin is treated with use of an amine compound having a phosphinic acid group or a phosphonic acid group, or with use of a salt of the amine compound (Patent Literature 6); a technique in which a water-absorbing resin is treated with use of an oxygen-containing reducing inorganic salt (Patent Literature 7); and a technique in which a water-absorbing resin is treated with use of an ion sealing agent such as an aminocarboxylic acid (salt) (Patent Literature 8).